The invention is directed to a method for determining the spatial location and position of a reflector rod in relation to a marked ground point, notably when it comes to geodetic measurements in the field.
In geodetic measurements, the location of a ground point, such as a boundary stone, is generally determined with the aid of a reflector that is attached to a reflector rod at a certain height above the ground point. The height of the reflector above the ground point is measured manually in most cases today and, therefore, always constitutes a source of error. In the field, this reflector rod is posed vertically on the ground point. By rotating the rod about the vertical rod axis, the reflector, e.g., a reflector prism or a corner cube prism, is aligned with a tachymeter or with another device employed to carry out the measurements. Following alignment, the actual measurement of horizontal and vertical angles and of distances can be performed. The location of the ground point may be determined wrongly, inter alia, because of incorrect adjustment of the circular level on the reflector rod and improper rod placement on the ground point. Furthermore, perpendicular placement of the rod requires a certain amount of time and is problematic whenever objects, such as shrubs and trees, obstruct the view between the tachymeter and the reflector prism or when points are hidden from view.
Several methods and arrangements are known, with which the spatial location and direction of a reflector rod can be determined. For example, two reflector prisms are arranged on a reflector rod at a known distance. Measuring the distance from the tachymeter to each of the two prisms and the relevant horizontal and vertical angles defines a spatial straight line, on which a point (ground point) located at a certain distance to these prisms can be determined. As a result, this rod can be moved almost freely and signal a variety of different points. Likewise, the calculation model is overdetermined (number of measurements>number of unknowns), so that the measurement can be verified. The fact that the measurements for the two reflectors are carried out separately is a disadvantage, however. On the one hand, this procedure is time-consuming, while on the other a stable spatial location of the rod between two measurements cannot always be guaranteed. According to U.S. Pat. No. 5,512,905, clinometers are used to orient the reflector rod. For this purpose, two clinometers lying vertically to each other are attached to the rod. They are to determine the relevant rotations ω and κ in two directions. But the rotational angle can only be allocated with certainty if the rotation φ about the reflector rod axis is zero or known. This, however, can be achieved only if the reflector is accurately aligned with the tachymeter. Further, the two tilts of the rod are limited to the working range of the clinometer sensors, thus restricting the free spatial movement of the rod.
WO 01/09642 A1 describes two methods and an arrangement for determining the spatial positions and orientations of reflectors, wherein a cut-off corner of the prism acting as a pinhole diaphragm is provided at the center of a corner cube prism that serves as a reflector. A position-sensitive sensor unit is placed behind this cut-off corner. When a distance is measured, part of the light beam emitted from the tachymeter is imaged onto the surface of the sensor unit through the pinhole diaphragm acting as a pinhole camera.
This imaged light spot can be located on the position-sensitive sensor unit, and its position can be determined accordingly. The position of the light spot in the image plane of the sensor unit is dependent on the incident angles of the measuring beam emitted from the laser tracker in relation to the reflector prism.
According to this method, measurement data about the incidence angle of the light beam entering the reflector in relation to the object and/or measurement data about the reflector orientation in relation to the object are generated, and the position and spatial orientation of the object are calculated on the basis of the measurement data about the direction and path length of the light beam as well as on the basis of additional measurement data. This method is primarily used to measure moving objects with an attached reflector prism. Furthermore, the reflector can be constantly corrected vertically to the light beam of the laser tracker with the aid of controlled mechanics. Likewise, it is possible to correct measured values through the angle of incidence and the wavelength.
If the prism is fixed to the object, two rotations of the object in relation to the light beam can be identified. The third rotation, i.e., the rotation about the axis of the prism, cannot be determined in this way, which is a disadvantage of this method.
WO 99/49280 A1 describes another method for determining the location and rotational position of an object, such as leveling staffs and rods, etc. Here, the spatial orientation of a graduated rod or coded staff is determined by a picture record made in a measuring head. A coded staff inclined toward the recording system in every direction is imaged on the sensor of a CCD camera. Using appropriate image processing, a bar code arranged on the staff can be measured from the image content and the direction of the staff in the image can be determined. With the bar code and the direction, it is possible to determine five orientation parameters. Here again, a rotation of the staff about its longitudinal axis cannot be determined. In order to determine the rotation about the longitudinal axis of the staff, an additional code running around the staff must be provided. Another disadvantage is that the imaging optics used must image the bar code in a resolved manner, so that it can be measured in the image. If there are greater distances between the imaging optics and the bar code of the staff, it is no longer possible to image the bar code in a resolved way.
Another arrangement for establishing or defining measuring points in geodetic measurements is known from WO 90/12282 A1. Here, a camera with supplementary optics is arranged on a reflector rod next to a reflector prism. A measurement beam emitted from a measuring instrument penetrates the optics of the camera and is imaged on a position-sensitive sensor of a sensor unit. All three components of the inclination of a reflector rod are measured from the position of the imaged light spot and from the measuring data of two clinometers that are arranged vertically to each other. The fact that additional clinometers are required constitutes a disadvantage.